Location

Location ANSS

2019/04/28 00:58:50 61.399 -149.935 40.4 4.0 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2019/04/28 00:58:50:0  61.40 -149.93  40.4 4.0 Alaska
 
 Stations used:
   AK.FIRE AK.GHO AK.GLI AK.HIN AK.KLU AK.KNK AK.PWL AK.RC01 
   AK.RND AK.SAW AK.SKN AK.SLK AK.SSN AK.SWD AK.TRF AT.PMR 
   AV.STLK TA.M22K TA.M23K TA.M24K TA.O22K 
 
 Filtering commands used:
   cut o DIST/3.3 -40 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.08 n 3 
 
 Best Fitting Double Couple
  Mo = 1.72e+22 dyne-cm
  Mw = 4.09 
  Z  = 54 km
  Plane   Strike  Dip  Rake
   NP1       34    65   -95
   NP2      225    25   -80
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.72e+22     20     127
    N   0.00e+00      4      36
    P  -1.72e+22     69     295

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     5.22e+21
       Mxy    -6.48e+21
       Mxz    -5.78e+21
       Myy     7.74e+21
       Myz     9.60e+21
       Mzz    -1.30e+22
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ##############---#####              
              #########-----------------#-           
             #######--------------------###          
           #######----------------------#####        
          ######-----------------------#######       
         #####-------------------------########      
        #####-------------------------##########     
        ####---------   --------------##########     
       #####--------- P -------------############    
       ####----------   ------------#############    
       ####------------------------##############    
       ###------------------------###############    
        ##----------------------################     
        ##---------------------#################     
         ##------------------###########   ####      
          #-----------------############ T ###       
           #--------------##############   ##        
             ----------####################          
              ------######################           
                 ######################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.30e+22  -5.78e+21  -9.60e+21 
 -5.78e+21   5.22e+21   6.48e+21 
 -9.60e+21   6.48e+21   7.74e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190428005850/index.html
        

Preferred Solution

The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion and first motion observations is

      STK = 225
      DIP = 25
     RAKE = -80
       MW = 4.09
       HS = 54.0

The NDK file is 20190428005850.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2019/04/28 00:58:50:0  61.40 -149.93  40.4 4.0 Alaska
 
 Stations used:
   AK.FIRE AK.GHO AK.GLI AK.HIN AK.KLU AK.KNK AK.PWL AK.RC01 
   AK.RND AK.SAW AK.SKN AK.SLK AK.SSN AK.SWD AK.TRF AT.PMR 
   AV.STLK TA.M22K TA.M23K TA.M24K TA.O22K 
 
 Filtering commands used:
   cut o DIST/3.3 -40 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.08 n 3 
 
 Best Fitting Double Couple
  Mo = 1.72e+22 dyne-cm
  Mw = 4.09 
  Z  = 54 km
  Plane   Strike  Dip  Rake
   NP1       34    65   -95
   NP2      225    25   -80
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.72e+22     20     127
    N   0.00e+00      4      36
    P  -1.72e+22     69     295

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     5.22e+21
       Mxy    -6.48e+21
       Mxz    -5.78e+21
       Myy     7.74e+21
       Myz     9.60e+21
       Mzz    -1.30e+22
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ##############---#####              
              #########-----------------#-           
             #######--------------------###          
           #######----------------------#####        
          ######-----------------------#######       
         #####-------------------------########      
        #####-------------------------##########     
        ####---------   --------------##########     
       #####--------- P -------------############    
       ####----------   ------------#############    
       ####------------------------##############    
       ###------------------------###############    
        ##----------------------################     
        ##---------------------#################     
         ##------------------###########   ####      
          #-----------------############ T ###       
           #--------------##############   ##        
             ----------####################          
              ------######################           
                 ######################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.30e+22  -5.78e+21  -9.60e+21 
 -5.78e+21   5.22e+21   6.48e+21 
 -9.60e+21   6.48e+21   7.74e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190428005850/index.html
	

Magnitudes

ML Magnitude


(a) ML computed using the IASPEI formula for Horizontal components; (b) ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.


(a) ML computed using the IASPEI formula for Vertical components (research); (b) ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.

Context

The next figure presents the focal mechanism for this earthquake (red) in the context of other events (blue) in the SLU Moment Tensor Catalog which are within ± 0.5 degrees of the new event. This comparison is shown in the left panel of the figure. The right panel shows the inferred direction of maximum compressive stress and the type of faulting (green is strike-slip, red is normal, blue is thrust; oblique is shown by a combination of colors).

Waveform Inversion using wvfgrd96

The focal mechanism was determined using broadband seismic waveforms. The location of the event and the and stations used for the waveform inversion are shown in the next figure.
Location of broadband stations used for waveform inversion

The program wvfgrd96 was used with good traces observed at short distance to determine the focal mechanism, depth and seismic moment. This technique requires a high quality signal and well determined velocity model for the Green functions. To the extent that these are the quality data, this type of mechanism should be preferred over the radiation pattern technique which requires the separate step of defining the pressure and tension quadrants and the correct strike.

The observed and predicted traces are filtered using the following gsac commands:

cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.08 n 3 
The results of this grid search from 0.5 to 19 km depth are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0    55    40    90   3.24 0.1899
WVFGRD96    2.0    55    40    90   3.38 0.2562
WVFGRD96    3.0    55    40    90   3.43 0.2336
WVFGRD96    4.0   185    60   -55   3.41 0.2231
WVFGRD96    5.0    25    80    60   3.42 0.2423
WVFGRD96    6.0    25    85    55   3.43 0.2603
WVFGRD96    7.0    25    85    50   3.44 0.2724
WVFGRD96    8.0    25    85    55   3.51 0.2813
WVFGRD96    9.0   110    45    40   3.54 0.2981
WVFGRD96   10.0   110    55    45   3.56 0.3133
WVFGRD96   11.0   110    55    45   3.58 0.3265
WVFGRD96   12.0   110    55    45   3.59 0.3358
WVFGRD96   13.0   290    45    25   3.58 0.3453
WVFGRD96   14.0   290    45    20   3.59 0.3532
WVFGRD96   15.0   290    45    25   3.60 0.3605
WVFGRD96   16.0   290    45    20   3.61 0.3670
WVFGRD96   17.0   270    60   -35   3.64 0.3729
WVFGRD96   18.0   270    60   -35   3.65 0.3788
WVFGRD96   19.0   270    60   -35   3.66 0.3835
WVFGRD96   20.0   270    60   -35   3.68 0.3865
WVFGRD96   21.0   285    45    -5   3.67 0.3884
WVFGRD96   22.0   285    45    -5   3.68 0.3928
WVFGRD96   23.0   280    45   -10   3.69 0.3973
WVFGRD96   24.0   280    45   -15   3.71 0.4021
WVFGRD96   25.0   280    45   -20   3.72 0.4077
WVFGRD96   26.0   280    45   -20   3.73 0.4144
WVFGRD96   27.0   275    45   -25   3.74 0.4211
WVFGRD96   28.0   270    25   -25   3.75 0.4297
WVFGRD96   29.0   270    25   -25   3.77 0.4382
WVFGRD96   30.0   265    25   -35   3.78 0.4482
WVFGRD96   31.0   260    25   -40   3.79 0.4601
WVFGRD96   32.0   255    25   -45   3.81 0.4734
WVFGRD96   33.0   250    25   -50   3.82 0.4862
WVFGRD96   34.0   250    25   -55   3.83 0.5030
WVFGRD96   35.0   250    25   -55   3.84 0.5175
WVFGRD96   36.0   245    25   -60   3.85 0.5311
WVFGRD96   37.0   245    25   -60   3.86 0.5426
WVFGRD96   38.0   240    25   -70   3.88 0.5524
WVFGRD96   39.0   235    25   -75   3.89 0.5621
WVFGRD96   40.0   235    25   -70   4.00 0.5580
WVFGRD96   41.0   235    25   -70   4.01 0.5747
WVFGRD96   42.0   235    25   -70   4.02 0.5876
WVFGRD96   43.0   235    25   -70   4.03 0.5990
WVFGRD96   44.0   230    25   -75   4.04 0.6089
WVFGRD96   45.0   230    25   -75   4.05 0.6164
WVFGRD96   46.0   230    25   -75   4.05 0.6241
WVFGRD96   47.0   230    25   -75   4.06 0.6295
WVFGRD96   48.0   230    25   -75   4.06 0.6352
WVFGRD96   49.0   230    25   -75   4.07 0.6383
WVFGRD96   50.0   230    25   -75   4.08 0.6426
WVFGRD96   51.0   225    25   -80   4.08 0.6441
WVFGRD96   52.0   225    25   -80   4.09 0.6466
WVFGRD96   53.0   225    25   -80   4.09 0.6463
WVFGRD96   54.0   225    25   -80   4.09 0.6471
WVFGRD96   55.0   225    25   -80   4.10 0.6457
WVFGRD96   56.0   225    25   -80   4.10 0.6445
WVFGRD96   57.0   225    25   -80   4.10 0.6430
WVFGRD96   58.0   225    25   -80   4.10 0.6383
WVFGRD96   59.0   225    25   -80   4.10 0.6363

The best solution is

WVFGRD96   54.0   225    25   -80   4.09 0.6471

The mechanism correspond to the best fit is
Figure 1. Waveform inversion focal mechanism

The best fit as a function of depth is given in the following figure:

Figure 2. Depth sensitivity for waveform mechanism

The comparison of the observed and predicted waveforms is given in the next figure. The red traces are the observed and the blue are the predicted. Each observed-predicted component is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. A pair of numbers is given in black at the right of each predicted traces. The upper number it the time shift required for maximum correlation between the observed and predicted traces. This time shift is required because the synthetics are not computed at exactly the same distance as the observed and because the velocity model used in the predictions may not be perfect. A positive time shift indicates that the prediction is too fast and should be delayed to match the observed trace (shift to the right in this figure). A negative value indicates that the prediction is too slow. The lower number gives the percentage of variance reduction to characterize the individual goodness of fit (100% indicates a perfect fit).

The bandpass filter used in the processing and for the display was

cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.08 n 3 
Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated.
Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to thewavefroms. Each solution is plotted as a vector at a given value of strike and dip with the angle of the vector representing the rake angle, measured, with respect to the upward vertical (N) in the figure.

A check on the assumed source location is possible by looking at the time shifts between the observed and predicted traces. The time shifts for waveform matching arise for several reasons:

Assuming only a mislocation, the time shifts are fit to a functional form:

 Time_shift = A + B cos Azimuth + C Sin Azimuth

The time shifts for this inversion lead to the next figure:

The derived shift in origin time and epicentral coordinates are given at the bottom of the figure.

Discussion

Acknowledgements

Thanks also to the many seismic network operators whose dedication make this effort possible: University of Nevada Reno, University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Iris stations and the Transportable Array of EarthScope.

Velocity Model

The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:

MODEL.01
Model after     8 iterations
ISOTROPIC
KGS
FLAT EARTH
1-D
CONSTANT VELOCITY
LINE08
LINE09
LINE10
LINE11
      H(KM)   VP(KM/S)   VS(KM/S) RHO(GM/CC)         QP         QS       ETAP       ETAS      FREFP      FREFS
     1.9000     3.4065     2.0089     2.2150  0.302E-02  0.679E-02   0.00       0.00       1.00       1.00    
     6.1000     5.5445     3.2953     2.6089  0.349E-02  0.784E-02   0.00       0.00       1.00       1.00    
    13.0000     6.2708     3.7396     2.7812  0.212E-02  0.476E-02   0.00       0.00       1.00       1.00    
    19.0000     6.4075     3.7680     2.8223  0.111E-02  0.249E-02   0.00       0.00       1.00       1.00    
     0.0000     7.9000     4.6200     3.2760  0.164E-10  0.370E-10   0.00       0.00       1.00       1.00    

Quality Control

Here we tabulate the reasons for not using certain digital data sets

The following stations did not have a valid response files:

Last Changed Sat Apr 27 20:53:21 CDT 2019